A pulveriser mill has a rotary grinding table or yoke, known as a grinding ring, which in most applications is positioned below a stationary upper ring, known as a top ring. The grinding ring is configured to rotate about a vertical rotation axis whilst the top ring remains stationary. A number of grinding elements in the form of steel balls is provided between the top ring and the grinding ring in order to crush raw material fed into the mill in gyratory fashion. That said, the grinding elements may be fixed or may be free to precess. A passage or air port is provided between an outer periphery of the grinding ring and an inner surface of the housing of the mill. Air sweeps upward through the air port and transports fines (crushed raw material) to a classifier provided above the top ring.
A port ring or rotatable throat is provided in the passage and is mounted around the outer periphery of the grinding ring such that it is co-rotatable therewith. The throat includes a plurality of inclined, planar vanes which project radially outwardly and are angularly spaced apart such that openings are defined between the vanes to allow air to flow from below the grinding ring to above the grinding ring.
In conventional pulveriser mills, as air passes through the throat from a plenum chamber below the grinding ring, it undergoes rapid acceleration as well as a change in direction which creates a large pressure shock which is undesirable and gives rise to an increased pressure drop across the mill. As a result of an increased pressure drop, the mill consumes more energy which leads to a reduction in efficiency as well as a reduction in mill throughput.
The Applicant desires a pulveriser mill which at least alleviates the above drawbacks.